Eliminate tor_circmgr::Error::PendingCanceled.

On !325 (closed), speaking of tor_circmgr::Error::PendingCanceled, @Diziet says:

I looked at the code in tor-circmgr/src/mgr.rs and AFAICT this can only get dropped without a reply if the task in launch panics, or is somehow dropped by the executor.

Perhaps we should handle the former by catching the exception and sending an error on the oneshot, and treat the latter as an error of kind ReactorShuttingDown

I think this a good idea.